December 2011 to March 2012 there was an outbreak of HAdV A31 at our unit. We analyzed the outbreak, the possible routes of transmission, and the medical consequences.
We studied all patients who were admitted at CAST during the outbreak (defined as the period from the first to the last verified case of HAdV A31 infection: December 12, 2011 to March 22, 2012). We also studied all patients diagnosed with HAdV infection from
November 2011 to July 2012. Clinical data, and information of hospitalizations, visits to the out-patient clinic and the patient housing (mainly housing families with children), were retrieved from the medical records. We also collected information of hygiene and cleaning routines, and measures during the outbreak, respectively, from the staff at the ward.
4.1.1 Detection and phylogenetic analyses of HAdV
For detection of HAdV, a Taqman real-time PCR targeting the hexon gene was used, as previously described177,178. All HAdV strains identified during the outbreak, and the following 4 months, were typed. The HAdV type was determined as described by Allard et al179. Phylogenetic analysis of the hexon gene was conducted using primers and PCR programs according to Leruez-Ville et al73, with slight modifications presented in paper I.
For phylogenetic analysis of the E3 gene, a 2-step PCR amplification of almost the entire HAdV E3 gene was performed using 8 sets of different pairs of primers designed from the known sequence of the HAdV A31 reference strain (AM749299.1). Further details are outlined in paper I. Sequencing and phylogenetic analyses were performed at the Department of Virology, Umeå Universtity, Umeå.
4.1.2 Description of the outbreak
Nine patients with HAdV A31 were identified. The outbreak is illustrated in Figure 2. Several measures were implemented:
1. HAdV screening of all admitted patients on two occasions, and regular testing of those with symptoms,
2. closing and disinfecting the common kitchen (day 38 of the outbreak),
3. transfer of all HAdV positive patients from the ward (day 46 of the outbreak).
After these steps were taken, there were no new cases identified in the ward for a period of four weeks, although three patients in the out-patient clinic were found to be HAdV A31 positive. These patients had all been previously admitted at the ward, at the same time as HAdV A31 positive patients. Transmission in the ward was thought to be over, and the kitchen reopened (on day 59 of the outbreak). However, two more cases of HAdV A31 were diagnosed on day 78 and 101 respectively (Figure 2)
Figure. 2. Timeline of the human adenovirus (HAdV) outbreak. All samples were fecal samples except in Patient 5 (plasma sample). Day refers to the day after the first identified HAdV A31 case. The bars indicate admission at CAST or “other ward” (intensive care unit or department of infectious diseases).
4.1.3 Did transmission between the patients occur?
Phylogenetic typing was conducted in all nine cases of HAdV A31 infection. Sequencing the hexon gene showed 100% homology between the patient strains, but also to reference strains of HAdV A31, whereby transmission among the patients could neither be concluded or ruled out. We therefore sequenced the more diverse E3 gene, revealing that the patient strains contained identical mutations not present in the reference strains, hence suggesting a common source of transmission. The hexon gene has a relatively low genetic variability49. In another reported HAdV outbreak among HSCT recipients, sequencing revealed similar results, with homology between the patient strains as well as the reference strains74.
4.1.4 Possible routes of transmission
All patients with HAdV A31 had been admitted to the ward. The two last cases (patients 8 and 9) had no connection in time to the other identified HAdV positive patients in the ward, but no other possible interactions with HAdV infected patients (out-patient clinic or patient housing), were identified for those patients.
The following factors may have contributed to transmission of HAdV:
1. Although HAdV positive patients were isolated, transmission may have occurred indirectly through contaminated surfaces or fomites in the ward (HAdV contamination may have occurred before hygiene measures were implemented, and after reopening the kitchen).
HAdV has been shown to remain infectious on surfaces for up to four weeks, and HAdV RNA can be detectable on surfaces for 3 months63,64. Monitoring of HAdV DNA in the environment after cleaning may be a way of preventing insufficient cleaning and
environmental transmission of HAdV180,181. Houldcraft et al reported of a cluster of four patients with HAdV A31, who seemed to be part of a nosocomial transmission, which was supported by whole genome sequencing (WGS), although two of the patients had been admitted three years apart. Other HAdV A31 cases had been in the ward during this period of three years, and may have been part of the transmission, but in those cases WGS could not be performed49. Routine phylogenetic typing of HAdV strains in HSCT patients could be an important measure, enabling early discovery of HAdV transmission.
2. Testing for HAdV at the end of the outbreak was less frequent, and unidentified cases, with mild or no symptoms, may have contributed to the continued transmission among patients in the ward. Asymptomatic HSCT patients can have high concentrations of HAdV in stool59.
3. It is also possible that staff members transmitted the virus between patients, although none of them reported any clinical symptoms during the outbreak. At the beginning of the
outbreak, there was an excessive strain on the staff, due to several seriously ill patients in the ward, and hygiene routines may not always have been properly maintained. Short staffing has been reported to be a risk factor for transmission of pathogens between patients182,183.
4. The ward used at the time was not optimal for infection control purposes, since some rooms did not have separate toilet and shower facilities. This has now been changed with the move to a new purpose-built transplant ward.
4.1.5 Clinical consequences
Transplantation was postponed due to HAdV, and later cancelled, in one patient. Five patients were treated with cidofovir because of high levels of viremia, but no patient
developed severe disease. Seven patients had concomitant aGVHD grade II–IV, complicating the clinical picture, with difficulties discriminating symptoms of GVHD from symptoms of HAdV.
4.1.6 Conclusion
An outbreak of HAdV among HSCT patients can be difficult to control. The clinical
consequences were significant for some patients, although no patient developed severe HAdV disease. Contamination of the common facilities in the ward, and unidentified asymptomatic cases, may have contributed to the prolonged outbreak.